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A research group at the Technical University of Denmark has discovered a new and stable ion conducting material by merging two known materials, one of which is intrinsically not stable, Bismuth oxide. The Bismuth-based material has a 100 times faster ion conductivity than all previous known solid-state oxygen ion conductors.

A research group at the Technical University of Denmark (DTU), Department of Energy Conversion and storage (DTU Energy) has discovered a new way to stabilize an ion conducting material with a 100 times faster ion conductivity than all previous known ion conductors.

Their results were published very recently in Nature Materials.

“The new Bismuth-based material shows an exceptional high chemical stability in reducing conditions and after redox cycles. The high conductivity is maintained and stays stable in environments, which were not possible before”, says Professor Nini Pryds, head of the research in electro functional materials at DTU Energy and one of the co-authors of this paper.

"Researchers have tried to stabilize Bismuth oxide for almost a century, but failed. It took us almost three years, but we succeeded due to our great knowledge in growing high quality thin films "

Prof. Nini Pryds, DTU Energy

The strength of bismuth oxide stands in its defects. Particularly, the material can hold a very large amount of mobile oxygen defects which are the basic charges to make the building blocks of ionic devices such as to name only a few can be, fuel cells, electrolysis cells, batteries, gas sensors, light photo-catalysts and ferroelectric materials in electronics.

“Analogous to the best metallic conductors such as copper or silver where the current is transported by electron, in δ-Bismuth oxide which is the best known ionic conductor, the current is transported by oxygen ions. There has been enormous interest over the years to use this material in application however; its instability prevented its use. What we did is stabilized this materials by tuning at the nano-scale with another stable material and made it work!” explains Professor Nini Pryds.

Despite the fact that δ-Bismuth oxide is made of just 2 elements (bismuth and oxygen), this compound can appear in different forms so it took several centuries for the mineralogist to identify the compound. The so-called δ-Bi2O3 is one these compounds, which exhibit the highest ionic conductivity and was discovered by L.G. Sillén (1916-1970) (Mineralogist, Swedish Royal academy) who firstly recognized the importance of the oxygen defect in the structure. However, its major problem is that it is highly unstable material and therefore could not be use, at least not until now!

Therefore, many researchers all over the world have tried repeatedly for many years to extend the stability of this compound with partially success. But the researchers from DTU took another innovative path and stabilized the highly unstable δ-Bi2O3 by making atomically thin multilayered structure of Er2O3-stabilized δ-Bi2O3 (ESB) and Gadolinium oxide (Gd2O3) doped Ceria (CeO2) (GDC). This created a “new” type of superstructure material with enhanced chemical stability, and which can operate under harsh atmosphere, which previously was not attainable for the individual ESB.

The discovery has been published in the renown scientific journal Nature Materials with the title ”Enhancement of the chemical stability in confined δ-Bi2O3”, wherein the DTU-researchers describe in detail how they used advanced Pulse Laser Deposition (PLD) techniques to create atomically flat layered multi structure, with individual layers of approximately 3 nanometers each.

“Researchers have tried to stabilize Bismuth oxide for almost a century, but failed. It took us almost three years, but we succeeded due to our great knowledge in growing high quality thin films and our knowledge in ionic and electronic transport mechanism in these films”, says Professor Nini Pryds.

The results of this work were recently patented, with the hope that this discovery opens brand new possibilities for using δ-Bismuth as an ion conductor.

“We have used very advanced fabrication and characterization methods together with our previous knowledge to create this thin films architecture. The use of thin film technology for Bi oxide is not trivial and required long and hard studies. That might be the reason why we succeeded, where many didn’t”, says senior researcher Vincenzo Esposito, who is one of the authors of this paper together with colleague and researcher Simone Sanna.

“…. three long and intense years of work using advance state-of-the-art characterization techniques combined with great team work results in these exciting results”, continues Dr. Simone Sanna.

Not only that the highly valued δ-Bismuth has been stabilized, the researchers of DTU Energy also managed to give the material new properties, which it didn’t have before. And by doing so they opened a door to a universe of new and unknown possibilities.

“We have created and tested the material, how the new Bismuth-based material will be used by the industry has to be explored. What we know is that they have gained a new way to access the best ion conductor available. And this concept can be further developed for many other technologies. We are currently exploring some of them”, says Vincenzo Esposito.

The researchers are now working moving onwards toward further fundamental studies of the material itself and new exciting way to use the unlocked forbidden material.